Recently, a microplasma jet is drawing attention due to its wide applicability, and has been realized by various power sources and electrode structures. The microplasma is characterized by a minute spatial size. In order to produce and keep plasma in a minute space, the medium density must increases so as to ensure the sufficient collision frequency between electrons/ions and atomic molecules of medium gas (plasma-producing gas). Therefore, the production of microplasma requires medium gas in the vicinity of an atmospheric pressure, i.e., medium gas with a density of about 1018 to 1022 cm−3.
Furthermore, generally, in the case of plasma on a conventional microscale, an electron temperature Te and a gas temperature Tg in the plasma reach almost the thermal equilibrium along with the increase in a working pressure, so that such plasma is called thermal equilibrium plasma. In contrast, in a region of microplasma, which has a size of μm scaled down from several mm, the energy is not transferred sufficiently by the collision between particles because of a shortened duration period τd of medium gas molecules in the plasma, and the non-equilibrium state of Te>>Tg is considered to be obtained as in low-pressure plasma.
Conventionally, a microplasma jet is produced in most cases by an afterglow system using plasma with a temperature thereof decreased. According to the afterglow system, plasma at a relatively high temperature produced inside a quartz pipe through which medium gas flows is pushed by a medium gas stream and blown out from the tip end of the pipe.
For example, according to a system described in Patent Document 1, argon (Ar) gas used as medium gas for producing plasma is allowed to flow into a quartz pipe and is jetted from a jet port. A coil is placed around the quartz pipe and a high-frequency current is induced to flow therethrough, whereby an induction electric field is generated in the quartz pipe. Argon atoms of the argon gas flowing into the quartz pipe are ionized in the induction electric field or magnetic field to become plasma at a high temperature (6,000 to 7,000° C.), and the plasma thus produced is pushed by the flow-in pressure of the argon gas to be jetted to the atmosphere from the jet port at the tip end of the quartz pipe. The jetted plasma generates a microplasma jet without being diffused due to the presence of the atmosphere.
On the other hand, as a system different from those described above, a system as shown in FIG. 11 is known, which has been proposed by Engemann et al. of Wuppertal University in Germany. In FIG. 11, reference 1 denotes a gas supply tube made of a quartz pipe with an inner diameter of about 2 to 5 mm, and helium gas having passed through an internal hollow thereof is jetted from a jet port 1a. A pair of coaxial electrodes 3a, 3b for producing plasma are placed at upstream and downstream positions on the outer circumference of an end of the gas supply tube 1 on the jet port 1a side. A low-frequency pulse voltage of about 10 kHz (for example, 6-12 kV, 13 kHz) is applied to the electrodes 3a, 3b to cause pulse discharge by a voltage applying unit 4, with the electrode 3a being at a ground potential and the electrode 3b being at a high potential, whereby a plasma jet (hereinafter, which also may be referred to as a low-frequency (LF) plasma jet) extending in an elongated shape from the jet port 1a is generated.
The LF plasma jet has unusual features in two aspects. First, unlike a plasma jet according to the afterglow system, a plasma jet that extends in an elongated shape and has a large ratio of a length to a diameter (i.e., aspect ratio) is obtained, and the jetting direction is determined in accordance with the direction of a voltage to be applied to the electrodes. More specifically, when the direction of a voltage to be applied to the electrodes is inverted, the jet extends in an opposite direction, i.e., in an upstream direction of gas. Furthermore, according to the high time resolution measurement, columnar discharge is not maintained, and a spherical plasma bullet is moving at a very high speed of 10 km/s, which is about 10,000 times that of a medium gas stream, in synchronization with the power source frequency. Thus, the production mechanism is not directly related to the medium gas stream.
Unlike the afterglow jet, in the plasma jet according to the above system, a medium gas stream is ionized to become plasma, so that the plasma can be radiated directly to an object. Furthermore, in the LF plasma jet, a pulse-shaped plasma bullet is jetted. Therefore, non-equilibrium in terms of time is created, i.e., a thermal non-equilibrium state is created since the energy cannot be transferred to neutral gas at each moment. The thermal non-equilibrium plasma can radiate a high-energy component without raising the temperature of the object.    Patent document 1: JP 2006-60130 A